One of the first tasks of a video circuit receiving a conventional analog video signal is to synchronize itself with respect to the frequency and the phase of the video signal. This synchronization task is performed by a phase-locked loop.
A phase-locked loop conventionally comprises a controllable oscillator delivering the reference signal (which will synchronize itself with the video signal) and a phase comparator whose role is to extract the position of the horizontal synchronization pulses from the video signal and to translate this position into the form of an error signal. The error signal provides the phase gap between the video signal and the reference signal. After passing through the loop filter, the error signal controls the oscillator so that the latter shifts in frequency for ensuring synchronization of the reference signal with respect to the video signal. In this instance, it is with respect to the horizontal synchronization pulses of the video signal.
Recognition of the horizontal synchronization pulses is made difficult by the very large diversity of signal properties which have to be supported. Thus, the signal/noise ratio of these signals can vary from more than 50 db in the best cases to around 0 db, or even less, for the worst cases in which the phase-locked loop has to operate. The approaches generally used to extract synchronization are based essentially on analog processes utilizing multipliers and integrators.